For many years, cylindrical print rolls have been used to print lettering and other images on sheets of corrugated board. To create the desired image, a printing die is first assembled and attached to a print carrier sheet, which is then attached to the print roll. As the print roll rotates, the printing machine applies ink to the printing die, which is transferred to the sheets of corrugated board as the sheets are conveyed through the printing machine and across the printing die. The carrier sheets are removable, so that they can be changed out for different jobs. In general, print carrier sheets carrying different dies are constructed as needed, and stored in a convenient location when not in use. Each print shop typically stores scores of printing dies for its various customers.
In a printing machine of this type, a number of different systems have been used for removably attaching the print carrier sheets to the print rolls. The Matthews Fast Lock system, for example, includes a static aluminum frame bolted into a longitudinal slot in the print roll. The frame includes two parallel longitudinal rails or clips adjacent to each other and spaced a short distance apart. The print carrier sheet includes an edge clip, commonly referred to as a “J-bar,” that attaches to one of the rails. On the other end, the print carrier sheet carries a number of elastic straps that clip to the other rail of the frame. 
The Matthews Fast Lock system described above, which has been widely used for many years, experiences some distinct disadvantages. First, the elastic bands inherently allow a certain amount of play in the carrier sheet attachment, which can adversely affect the consistency of the registration of the printed image on the sheet or blank being printed. Second, and more problematically, an elastic strap may occasionally break or lose a clip. The loose strap or clip may then destroy the printing die or another component of the machine, such as an expensive anilox roll. Even if the loose clip or strap does not cause damage to the printing machine or the printing die, the machine must still be shutdown in the middle of the run to remove the loose items and replace the damaged strap. Of course, shutting down the printing machine in the middle of a run is costly in terms of lost time and wasted product.
Full-wrap systems have been developed to overcome the problems associated with elastic straps described above. Full-wrap systems use print carrier sheets that wrap almost all the way around the print roll and, for this reason, do not use elastic straps or the associated clips. However, these systems have their own problems. The Inland-Annen system, for example, includes a stationary clip for attaching to an elongated J-bar on one side, just like the Matthews Fast Lock system. But for the other side of the print carrier sheet, instead of a stationary rail the Inland-Annen system includes a narrow roller inset into the print roll. This narrow roller receives a plastic strip that is typically sewn onto the other end of the full-wrap carrier sheet. A crank is turned to rotate the roller and thereby tighten the carrier sheet on the print roll. Although this system secures the print carrier sheets without the use of elastic bands, the roller system tends to crack and wear out the end of the carrier sheet that wraps around the roller due to the tight turning radius of the roller. In addition, tightly rolling the end of the carrier sheets causes the sheets to remain tightly curled after they are removed from the print roll, which makes storage difficult.
The Bobst Group has developed a full-wrap system that does not rely on a roller to tighten the print carrier sheer on the print roll. Instead of a roller, the Bobst machine includes a hydraulically operated sliding jaw system to tighten the print carrier sheer on the print roll. Although effective, this system is very expensive and difficult to install. As a result, the hydraulic jaw system is not economically feasible for many applications and, in general, is not suitable as a retrofit for an existing machine that includes a Matthews Fast Lock system. 
Accordingly, there is an ongoing need for a cost effective lock-up system for attaching print carrier sheets to print rolls. There is a further need for a lock-up system that is suitable as a retrofit for an existing machine that includes a Matthews Fast Lock system.